THE OSTEOLOGY OF AMIURUS CATUS. 291 



subcutaneous system, and the latter to the system of the branchios- 

 tegal rays. Hollard 1 sums up his observations thus, " En d'autres 

 termes et pour nous re'sumer, il resulte pour nous de cette e*tude que 

 le battant operculaire des Poissons se divise, quant a sa signification 

 anatomique. entre le squelette normal et un squelette supple'mentaire 

 et cutane ; que l'interopercule appartient au premier, com me naissant 

 et se developpant dans le premier arc visceral ; qu'il occupe la m§me 

 place que l'enclume des mammiferes ; qu'enfin l'opercule et le sous- 

 opercule, loin de lui faire suite, loin de pouvoir §tre assimiles aux 

 autres osselets de l'ouie ou a vrais appendices, sortent des limites du 

 nevro-squelette, non, comme le voulait Cuvier, a titre de pieces sans 

 analogues mais en se rattachant au developpement si gdneral et si 

 considerable des expansions tdgumentaires des Poissons." Owen 2 

 does not commit himself definitely either way, considering them 

 merely appendages to the " tympano-mandibular arch," but however 

 implies a certain amount of credence in the auditory theory, by 

 referring them to the mandibular rather than to the hyoid arcade. 

 Lastly, Gegenbaur 3 suggests that the interoperculum was originally 

 a part, not of the hyoid skeleton, but of the mandibular. 



It is now a recognized fact that the homologues of the auditory os- 

 sicles are not to be looked for in the opercular bones, and we have 

 remaining the theories that they are a subcutaneous system, a part of 

 the branchiostegal system, and that the interoperculum is a part of 

 the mandibular arcade. In Amiurus they seem to belong to the 

 branchiostegal system, with the exception of the preoperculum. This 

 is formed round a mucous canal, and is one of what may be called 

 the mucous canal series, to which also the infraorbital ossicles belong. 

 Functionally it is not one of the opercular bones but protects the in- 

 cluded mucous canal. The suboperculum is properly a bone lying 

 below the lower edge of the operculum. This is the position it holds 

 Esox, also in Salmo, but in the latter case it is increased in size, and 

 projects largely from under the operculum. In both these forms 

 also it lies on the inner side of the interoperculum. In Amiurus, 

 what is usually considered the upper branchiostegal ray bears exactly 

 the same relations. Shortly behind its attachment to the epihyal, 



1 Hollard.- De la signification de l'appareil operculaire des Poissons. Ann. des Sei. Nat.> 

 1864. 

 s Owen. — On the anatomy of the vertebrates. Vol. I., Loudon, 1866. 

 3 hoc. ait.' 



